Recuce Sign and Encrypt to single split button
* src/dialogs.h: Add new icon ids. * src/ribbon-callbacks.h: Add new ids. * src/ribbon-callbacks.cpp (GpgolRibbonExtender::GetIDsOfNames), (GpgolRibbonExtender::Invoke): Handle new ids. (GetCustomUI_MIME): Use split button. * src/ribbon-callbacks.cpp (mark_mime_action): Invalidate ui. (get_crypt_pressed): Handle queries for multiple flags.